Ive had heartburn this whole time not that kind of burning. Feels like muscle/skin is on fire type. Not internal
After reading your question again about the on and off cramping and spasm feeling makes me wonder if it is contractions or Braxton Hicks. ... keep track of how often your getting them.
All sounds normal to me. The burning may be heartburn, cramping and back aches are very normal at 36w....They might be contractions. . your getting close.